1. Field of the Invention
The present invention relates to an imaging optical system, a camera and a personal digital assistant.
An imaging optical system of the present invention can be used as a lens system of a digital camera and a digital video camera, and also can be used as a lens system of a silver salt camera.
Therefore, a camera of the present invention can be used as a digital camera, a digital video camera and a silver salt camera having the imaging optical system of the present invention. A personal digital assistant of the present invention can be used as a personal digital assistant which has a shooting section of the camera of the present invention and can perform information processing using a communication system and the like.
2. Description of the Related Art
In recent years, the use of digital cameras has widely increased, their performances have improved, and their types have diversified. Under such circumstances, many users require a high image quality compact digital camera having a high-performance single focus lens, and also require a compact camera having a small F-number and a large-diameter lens in addition to a high performance.
Meanwhile, in order to use a camera having a small F-number in a lighted environment such as on a fine day, it is necessary to use a high-speed shutter unit.
In many general lens-integrated digital cameras, a shutter is disposed close to an aperture stop, and a smaller F-number, i.e., a larger aperture stop, requires a high-speed operation of a shutter unit. For this reason, in a compact camera having a large-diameter lens, the size of a shutter unit tends to increase, and also the power consumption tends to increase.
In order to avoid such problems, JP2006-301473A, JP2000-305018A, JP2007-114283A and JP2008-028963A propose the use of an ND filter (neutral density filter) for attenuating light amount. According to the inventions proposed in JP2006-301473A, JP2000-305018A, JP2007-114283A and JP2008-028963A, an ND filter is built in a shutter unit, this ND filter is configured to be inserted and retracted on and from the optical axis of the imaging optical system. In general, when the ND filter is not necessary, the ND filter is retracted from the optical axis, and when the ND filter is necessary, the ND filter is disposed on the optical axis, so as to cover the entire aperture stop, and then the shutter is activated in a state in which the shooting light amount is attenuated.
By constituting as described above, appropriately exposed shooting can be performed in a lighted environment without much increasing a shutter speed.
In addition, the shooting light amount can be attenuated without reducing the diameter of the aperture stop, so that the deterioration in the optical performance by the light diffraction generated by reducing the diameter of the aperture stop can be controlled. Moreover, since the diameter of the aperture stop can be increased, shooting with a narrow depth of field can be performed.
Furthermore, by placing the ND filter close to the aperture stop, a driving source and its power source, which are required for inserting and retracting the ND filter on and from the optical axis, can be shared with those of the shutter unit. Also, by placing the ND filter close to the aperture stop, the ND filter covers the entire shooting light flux, so that a large diameter ND filter becomes unnecessary.
However, by placing the ND filter close to the aperture stop, a ghost image may occur by the reflected light from an imaging element side.